sludge dewatering - snf

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SLUDGE DEWATERING

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Page 1: sludge dewatering - snf

SLUDGEDEWATERING

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INDEXSludge characterisation: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41.1. Origin of the sludge: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41.2. The different types of sludge: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

1.2.1. Primary sludge: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41.2.2. Biological sludge: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41.2.3. Mixed sludge: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51.2.4. Digested sludge: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51.2.5. Physico-chemical sludge: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .61.2.6 Mineral sludge: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

1.3. Parameters that influence the dewatering abilities of sludge: . . . . . . . . . . . . . . . . . . . . . . . .61.3.1. Concentration (g/l): . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .61.3.2. The organic matter content (%): . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .61.3.3. The colloidal nature of the sludge: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

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Dewatering aids: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72.1. Mineral chemicals: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1.1. Iron salts: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72.1.2. Lime: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

2.2. Organic chemicals: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .82.2.1. Flocculation mechanism: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .82.2.2. Destabilized particles: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

2.3. Parameters of the organic chemicals that will influence dewatering: . . . . . . . . . . . . . . . . .92.3.1. The type of charge ( + or - ): . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .92.3.2. The charge density (%): . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .92.3.3. The molecular weight (MW): . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .102.3.4. The molecular structure: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .102.3.5. The type of monomer: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

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Dynamic thickening: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.1. Flotation: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.1.1. Indirect flotation: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.1.2. Direct flotation: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.1.3. Sludge conditioning before flotation: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.2. Thickening table: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.2.1. Operating principle: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.2.2. Sludge conditioning before thickening table: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.3. Thickening drum, screw drum: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.3.1. Operating principle: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.3.2. Sludge conditioning before thickening drum: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.4. Centrifuge: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.4.1. Operating principle: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.4.2. Sludge conditioning before centrifuge: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

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Today, the treatment of water is a well-known process and is executed by state of the art techniques. The sludge resulting from this process represents the next challenge for the water treatment industry, inparticular the minimizing of its volume.This Sludge Dewatering handbook will present the key parameters to take into account in order to optimi-ze sludge treatment with SNF Floerger’s organic polymers.

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Belt filter dewatering: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.1. Equipment description and operating principle: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.2. Lab tests: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.2.1. Sampling: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.2.2. Laboratory equipment: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.2.3. Test procedures: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184.2.4. Parameters to check and analysis of the results: . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.3. Plant trials: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.3.1. Setting up a trial: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.3.2. Parameters to follow and analysis of results: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.4. Optimizing belt filters: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.4.1. Bad drainage: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.4.2. Sludge creeping: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.4.3. Low cake dryness: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.4.4. Summary of the adjustable parameters: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Centrifuge dewatering: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.1. Equipment description and operating principle: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.2. Lab tests: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5.2.1. Sampling: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235.2.2. Laboratory equipment: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245.2.3. Test procedures: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245.2.4. Parameters to check and analysis of the results: . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

5.3. Plant trials: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255.3.1. Setting up a trial: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255.3.2. Parameters to follow and analysis of results: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

5.4. Optimizing centrifuges: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265.4.1. Black centrate: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265.4.2. Gray, foaming centrate: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265.4.3. Low cake dryness: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.4.4. Summary of the adjustable parameters: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

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Frame filter press dewatering: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286.1. Equipment description and operating principle: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286.2. Lab tests: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

6.2.1. Sampling: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306.2.2. Laboratory equipment: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306.2.3. Test procedures: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306.2.4. Parameters to check and analysis of the results: . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

6.3. Plant trials: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326.3.1. Setting up a trial: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326.3.2. Parameters to follow and analysis of results: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

6.4. Optimizing frame filter press: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326.4.1. Sticky cakes: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336.4.2. Cloth plugging: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336.4.3. Polymer efficiency loss: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

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Sludge characterisation1

1.1. Origin of the sludge:

During the course of the water treatment, products coming from the pollution are extractedwhile the treated water is released in the environment.

Amongst these products coming from the pollution one can distinguish:

● Particles that decant naturally or that come from the physico-chemical treatment● Excess micro-organisms coming from the dissolved organic matter treatment● Mineral matter that is non biodegradable

All these products are suspended in more or less concentrated forms and the resulting liquidis called sludge.

1.2. The different types of sludge:

1.2.1. Primary sludge:

Primary sludge comes from the settling process. It is therefore made of easily decantablesuspended particles: large and/or dense particles.It has a low level of Volatile Solids content (VS around 55% to 60%) and its dewatering ability is excellent.It is also very easy to concentrate this type of sludge with a static thickening step just befo-re dewatering. The drawback is that this sludge ferments very easily.

1.2.2. Biological sludge:

Biological sludge comes from the biological treatment of the wastewater. It is made of amixture of microorganisms. These microorganisms, mainly bacteria, amalgamate in bacterial flocs through the synthesis of exo-polymers. A simple decantation in the clarifierwill easily separate the bacterial flocs from the treated water.Only part of this settled sludge is sent to dewatering: the excess biological sludge; part of itis recirculated to maintain the bacterial population in the reactor.

S L U D G E D E W A T E R I N G

There are several types of sludge that have specific characteristics that will influence:● The choice in conditioning chemical (cationic flocculant, ferric chloride, lime…)● The choice in the dewatering equipment to be used (filtration, centrifuge…)

These choices will also depend on the final use of the sludge (incineration, agricultural spreading…)

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To simplify, we will not differentiate between the different qualities of biological sludge (pro-longed aeration, low charge, high charge…); their main properties are:● A high Volatile Solids content: VS around 70% to 80%.● A low dry solids content: 7 g/l to 10 g/l. It is often necessary to introduce a dynamic

thickening step by flotation or gravity belt.● The dewatering ability is medium. It depends partially on the VS. The higher the VS the

harder it is to extract the water from the sludge.

1.2.3. Mixed sludge:

Mixed sludge is a blend of primary and biological sludges. The blending ratio is often asfollows: ● 35% to 45% of primary sludge. ● 65% to 55% of biological sludge.

This blending will permit an easier dewatering as the intrinsic properties of this sludge arebetween the other two types.

1.2.4. Digested sludge:

Digested sludge comes from a biological stabilizing step in the process called digestion. Thisstabilization is performed on biological or mixed sludge. It can be done under different temperatures (mesophilic or thermophilic) and with or without the presence of oxygen (aero-bic or anaerobic). Following this stabilization step the properties of the sludge are:● A lower Volatile Solids content: VS around 50%. A mineralisation of the sludge occursduring digestion● A dry solids content around 20 g/l to 40 g/l● A good dewatering ability.

1.2.5. Physico-chemical sludge:

This type of sludge is the result of a physico-chemical treatment of the wastewater (seebrochure “Coagulation Flocculation”). It is composed of flocs produced by the chemical treat-ment (coagulants and/or flocculants).The characteristics of this sludge is the direct result of the chemicals used (mineral or organiccoagulants) and of course of the pollutants in the water.

1.2.6. Mineral sludge:

This name is given to sludge produced during mineral processes such as quarries or miningbeneficiation processes. Their nature is essentially mineral particles of various sizes (includingclays). They have a very good aptitude to settle by gravity and very high concentrations arefrequently obtained.

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1.3. Parameters that influence the dewatering abilities of sludge:

Several parameters concerning the sludge will influence its ability to dewater easily. Amongst these, the main ones are:

1.3.1. Concentration (g/l):

Measured in g/l, the concentration of the sludge will influence:

● The incorporation of the flocculant. The higher the concentration of the sludge, the harderit is to mix in a viscous solution of flocculant (even at low flocculant concentrations). Solutions to this problem are: post-dilution of the flocculant, injecting the flocculant upstream, multiple injection points of the flocculant, use an on-line mixer.

● The consumption of flocculant. The higher the concentration of the sludge, the lowerthe consumption of flocculant. This is true only if the incorporation is correctly done.

1.3.2. The organic matter content (%):

The organic matter content is comparable to the Volatile Solids content (VS).

The higher the VS, the more difficult the dewatering. The dryness achieved will be low, themechanical properties will be low and the flocculant consumption will be high.

When the VS of the sludge is high, it is recommended to add a thickening step in the processin order to achieve a better dewatering.

1.3.3. The colloidal nature of the sludge:

This characteristic has a very important effect on the dewatering performance. The higherthe colloidal nature, the more difficult it is to dewater.Four factors will affect the colloidal nature of the sludge:

● The origin of the sludge:

Primary Digested primary Fresh mixed Digested mixed BiologicalLow colloidal nature High colloidal nature

● The freshness of the sludge: the colloidal nature of the sludge will increase with its level of fermentation (septic sludge).

● The origin of the wastewater: a dairy or brewery origin will increase the colloidal nature of the sludge.● The sludge return: a badly controlled return of the sludge will increase its colloidal nature.

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T E R I N G

Sludge is generally conditioned before thickening and dewatering. Two types of conditioningchemicals are used to enhance the treatability of the sludge:● Mineral chemicals such as iron salts and lime. These chemicals are frequently found in

filter press applications.● Organic chemicals such as coagulants and flocculants. The most common type of

flocculants encountered are cationic in nature.

2.1. Mineral chemicals:

2.1.1. Iron salts:

Ferric Chloride and Iron Chloro-Sulfate are mainly used in conjunction with lime to condi-tion the sludge before a filter press.They allow a better filterability by coagulating the colloids (thus lowering the content of lin-ked water) and by micro-flocculation of the precipitates (hydroxides).The dosages for iron salts are between 3% and 15% of the dry content, depending on thequality of the sludge.

There is a trend to associate iron salts with organic flocculants (cationic) in order to lowerthe volume of the sludge produced compared to a classic iron salts + lime process.

2.1.2. Lime:

Lime as a conditioning agent is only used in conjunction with iron salts on filter press appli-cations. It brings a mineral nature to the sludge and strengthens its mechanical properties(higher specific resistance to filtration).The dosages for lime are between 15% and 40% of the dry content.

Remarks:

● Lime is also used after dewatering to stabilize the sludge.● The specific resistance to filtration (r) depends on the size, shape and degree of

agglomeration of the solid particles that make-up the cake from a filter-press. It isindependent of the sludge concentration.

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2.2. Organic chemicals:

Cationic flocculants represent the large majority of the chemicals used in sludge dewatering.

2.2.1. Flocculation mechanism:

Flocculation of sludge is the step in the process where destabilized particles are agglo-merated in aggregates called flocs.

Flocculants, with their very high molecular weights (long chains of monomers) and theirvaried ionic charge, fix the destabilized particles on their chain. Therefore the particle size in the aqueous phase will increase throughout the flocculation step with theformation of flocs.The formation of flocs induces a release of the water. This water will thus be easily eliminatedduring the dewatering step.

2.2.2. Destabilized particles:

The origin of destabilized particles varies a lot and essentially depends on the nature of thesludge.The charge that the flocculant brings will be selected according to the type of destabilizedparticles present in the sludge to be treated. It will therefore depend on the type of sludge(biological, digested, physico-chemical, mineral… see paragraph n°1).The charge to be brought often follows the pattern below:

● Low to medium anionic for mineral sludge.● Low anionic to low cationic for physico-chemical sludge.● Low cationic for digested and primary sludge.● Medium cationic for mixed sludge.● High cationic for biological sludge.

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2.3. Parameters of the organic chemicals that will influence dewatering:

Organic flocculants are characterised by five main parameters:

● The type of charge● The charge density● The molecular weight● The molecular structure● The type of monomer

These will influence the quality of the flocculation and thus the quality of the dewatering.

2.3.1. The type of charge (+ or -):

The type of charge of a flocculant is selected according to the type of particles. Generallythe choice follows the pattern below:

- An anionic (-) flocculant to catch mineral particles.- A cationic (+) flocculant to catch organic particles.

As usual, only a laboratory test can really determine which charge is well adapted.

2.3.2. The charge density (%):

The charge density represents the quantity of + or – charge necessary to obtain the best floc-culation at the lowest dosage. The charge density depends on the type of sludge to treat.For municipal sludge, this charge density is mainly a function of the Organic Matter content(OM) in the sludge. The OM is generally assimilated to the Volatile Solids content (VS).The higher the VS, the higher the cationic charge needed.

9

T E R I N G

CATIONIC NON-IONIC ANIONIC

100% 100%0%

Paper industrysludge dewatering

Red mud thickeningin alumina

Most industrialwastewater

Sugarindustry

Coalwashing

Raw water clarificationfor drinking water

Alcaline muddewatering

Primary sludgedewatering

Digested sludgedewatering

Mixed sludgedewatering

Biological sludgedewatering

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10

2.3.3. The molecular weight (MW):

The choice of molecular weight, which is the length of the polymer chain, depends on thetype of equipment used for dewatering.For centrifuge: A high to very high molecular weight is best adapted due to the high shearingapplied to the flocs.For filtration: A low to medium molecular weight will be best adapted to obtain a good drainage.

2.3.4. The molecular structure:

The molecular structure of the flocculant depends on the dewatering performances required.For cationic flocculants there are:

● Linear structures: with low dosage and good performance when thecorrect molecular weight is chosen.

● Branched structures: with medium dosage and excellent drainageperformance.

● Cross-linked structures: with high dosage and exceptional drainage per-formance and shear resistance.

2.3.5. The type of monomer:

The type of monomer used to synthesize the flocculants also influence flocculation. Two different cationic monomers are commonly used:● ADAM-MeCl: see brochure “Preparation of organic polymers”.● APTAC: insensitive to hydrolysis of the cationic charges, they sometimes react better on deinking sludge from the paper industry.

The most frequent anionic monomer is sodium acrylate.

S L U D G E D E W A T E R I N G

BLOCKED CHARGES OF CROSS-LINKED POLYMERS

Accessible charges

Blocked charges

The floc structure ismaintained

++

++

++

++

++

+ +++

+++

+++

+ ++ + +

+

+

+

DIAGRAM OF THE SHEAR RESISTANCE

AGITATION

LINEAR OR CROSS-LINKED STRUCTURE ?

LINEAR

ADVANTAGES

DRAWBACKS

CROSS-LINKED

LINEAR CROSS-LINKED

� Low dosage � Very strong flocs � Large range of MW

� Low strenght of the flocs � Possibility of overdosing

� High dosage

� Excellent drainage � Higher cake dryness

++ +

++ +

++

++ +

+++ + + ++

+

+++

+++ +

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++

++

+ +++ ++ +

+++

+

++

+

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++

+

+

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311

Dynamic thickening of sludge is not systematic. When implemented, it is done:

● Before dewatering, in order to increase the Dry Solids Content and facilitate the dewatering step (less equipment, less reagents…)● Before farm spreading, to lower the volume thus lowering the number of truckloads.Four types of equipment are used to thicken sludge dynamically: flotation, gravity belt, drumfilter and centrifuge.

3.1. Flotation:

Flotation is mainly used to thicken biological sludge that comes from the clarifier.The principle is to attach micro-bubbles to the particles in the sludge. Air injection and pres-surisation create these micro-bubbles. The pressurisation can be applied on:

● The water underflow that is mixed to the sludge when depressurised (indirect flotation).With micro-bubbles fixed to the particles, these have then a lower density and float to thesurface. The thickened sludge is then evacuated in the overflow and the underflow water isrecycled at the beginning of the plant.● The raw sludge (direct flotation)

3.1.1. Indirect flotation:

T E R I N G

Floated sludge

Air

"White water"Relief valve

Compressor

Raw sludge

Underflow

Air saturationtank

Recirculationpump

Skimming zone

Heavysludge

Transition zone

Clarification zone

Contact zone

Settling zone

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Dynamic thickening3

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S L U D G E D E W A T E R I N G

3.1.2. Direct flotation:

3.1.3. Sludge conditioning before flotation:

Sludge conditioning before flotation with an organic polymer is not essential but is stronglyrecommended to get a better underflow by enhancing of the capture rate.

3.2. Gravity belt:

It is essential to use an organic flocculant on gravity belt. The flocculant will accelerate thedrainage of water and allow it to flow through the sludge and the filtration belt.

LAB TESTS PLANT TRIALS

● Floc size● Overflow quality● Floc formation speed● Shear resistance of the flocs

● Sludge flow● Polymer flow ● Injection point● Capture rate● Floating sludge concentration● Pressure

KEYPARAMETERS

LAB PROCEDURE/SPREADSHEET

Jar testmethod

Laboratory trials -CoagulationFlocculation

RECOMMENDATIONS

/ /

● Select the best charge density forthe sludge.● Select several molecular weights tobe tested industrially. Use a high mole-cular weight for direct flotation.

Floated sludge

Air

Relief valve

Compressor

Raw sludge

Air saturationtank

Heavysludge

Underflow

Skimming zoneTransition zone

Clarification zone

Contact zone

Settling zone

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● Polymer dosage should be bet-ween 0.2 and 1.0 kg/dry solidstonne (0,4 and 2 lbs/DT).● The injection point of thepolymer is a key element andthe capture rate will depend on it.

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T E R I N G

The flocculated sludge flows over a filtration belt that is conveyed at a certain speed. The water freed by the flocculation step is drained through the pores of the belt. This elimination of water (the filtrate) leads to a thickening of the sludge at the end of theconveyer belt. Picket fences resting on the belt are often used to enhance the gravity drainage.Continuous pressure cleaning of the belt is necessary to prevent pore plugging.The filtrate is returned to the beginning of the process while the thickened sludge is sent toa temporary storage tank before dewatering.

3.2.1. Functionning principle:

3.2.2. Sludge conditioning before gravity belt:

LAB TESTS PLANT TRIALS

● Drainage speed● Filtrate quality● Floc formation speed

KEYPARAMETERS

LAB PROCEDURE/SPREADSHEET

Laboratory trials -Gravity belt& Belt filter

Laboratory evaluationfor belt press

and gravity belt

Gravity belt& Belt filter

performance sheet

RECOMMENDATIONS

/

● Select the best charge density forthe sludge.● Select the molecular weight best suited for drainage.

● Polymer dosage should be bet-ween 3.0 and 10.0 kg/Dry SolidsTonne (6 and 20 lbs/DT).● The injection point of the poly-mer is a key element and the drai-nage speed will depend on it.● Check carefully the water line.

● Sludge flow● Polymer flow ● Injection points● Belt speed● Belt cleaning● Flocculator speed● Thickened sludge concentration● Filtrate quality

Sludge+

Polymer

Filtrate

Thickenedsludge

Flocculator1

1

Guide2

2

Picket fences3

3

Cleaning ramp4

4

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S L U D G E D E W A T E R I N G

3.3. Thickening drum, screw drum:

The principle is identical to the gravity belt: sludge conditioning with a flocculant, liberation of intersticial water in the sludge, gravity drainage of the free water through a grid.In the case of the screw drum, the only difference is that the sludge conveying is done withan Archimedean screw.

3.3.1. operating principle:

3.3.2. Sludge conditionning before thickening drum:

LAB TESTS PLANT TRIALS

● Drainage speed● Shear resistance● Filtrate quality● Floc formation speedKEY

PARAMETERS

LAB PROCEDURE/SPREADSHEET

Laboratory trials -Gravity belt

& Belt filter +Laboratory trials -

Centrifuge

Laboratory evaluationfor belt press

and gravity belt +Laboratory evaluation

for centrifuges

Gravity belt& Belt filter

performance sheet

RECOMMENDATIONS

/

● Select the best charge density forthe sludge.● Select the molecular weight best suited for drainage.

● Polymer dosage should be bet-ween 3.0 and 10.0 kg/Dry SolidsTonne (6 and 20 lbs/DT).● Check the thickened sludgequality.

● Sludge flow● Polymer flow ● Injection points● Screw speed● Grid cleaning● Thickened sludge concentration● Filtrate quality

Variable speed flocculator1

1

Cleaning ramp2

2

Sludge+

Polymer

ThickenedsludgeFiltrate

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T E R I N G

3.4. Centrifuge:

The centrifuge principle is completely different (from previous methods). With centrifugationit is the centrifugal force instead of gravity that is used to force the solid/liquid separation.This force is created in a conical-cylinder bowl that rotates at high speed (2500-3500 rpm).The sludge particles are pressed against the bowl and conveyed out of the centrifuge by ascrew that rotates at a slightly different speed than the bowl (a few rpms).

3.4.1. Operating principle :

3.4.2. Sludge conditioning before centrifuge:

LAB TESTS PLANT TRIALS

● Shear resistance● Filtrate quality● Floc formation speedKEY

PARAMETERS

LAB PROCEDURE/SPREADSHEET

Laboratory trials -Centrifuge

Laboratoryevaluation

for centrifuge

Centrifuge performance sheet

RECOMMENDATIONS

/

● Select the best charge density forthe sludge.● Select the molecular weight best suited for shear resistance of the flocs.

● Polymer dosage should be between 3.0 and 8.0 kg/Dry Solids Tonne (6 and 16 lbs/DT).● Check the thickened sludge quality.

● Sludge flow● Polymer flow ● Injection points● Relative speed of the screw and/or torque● Thickened sludge concentration● Filtrate quality

Filtrate or centrate1

1

1

Thickened sludge2

2

Bowl3

3

Screw4

4

Sludge+

polymer

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4Belt filter dewatering4

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S L U D G E D E W A T E R I N G

Belt Filters (Belt Filter Presses, BFP) allow for a continuous sludge dewatering between two filter belts.

4.1. Equipment description and operating principle:

There are many variations in belt filters, but all of them have the following characteristics:

● A flocculator: The sludge is conditioned before arriving in the drainage zone. Thesludge-flocculant blend is done in the flocculator and the flocculated sludge is distributedevenly on the filter belt. At this level the sludge is in the form of flocs with the free water inbetween the flocs.

● A gravity drainage zone: The flocculated sludge is drained on a first belt (lower belt) bysimple gravity. The drainage is helped by picket fences that lay freely on the belt. In this zonea water line is created that corresponds roughly to the moment where the majority of thewater freed by flocculation is eliminated.

● A progressive compression zone: The sludge, after drainage of the water freed byflocculation, is then pressed between two filter belts. With the arrival of the top belt, a progressive pressurization takes place:

- Up to 4 bars for low-pressure belt filters.- Up to 5 bars for medium pressure belt filters.- Up to 7 bars for high-pressure belt filters.

● A cake scraping zone: Once pressed, the sludge has a more solid aspect. It is called a sludgecake or simply cake. This cake is then scraped off from the surface of the two belts thatseperate at this level.

● A high pressurewashing station:A bank of nozzles under 7 to 8 bars (100 to 120 PSI)continuously cleans each belt.

Filtrate

Cake

Sludge+

polymer

Flocculator1

1

Picket fence2

2

Arrival of the top belt3

3

Progressive compression zone44Tracking roller5

5

Cleaning ramp6

6

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T E R I N G

4.2. Lab tests:

4.2.1. Sampling:

Of the solution to be treated.A representative sample of the sludge must be taken. In order to achieve this, the sample will be taken just before the polymer injection point and thelab tests will be executed rapidly after sampling (the sludge condition may change over time).An analysis of the Dry Solids content (DS) of the sludge must be made since the polymer dosageis a function of the DS.

Of the polymer.It is not necessary to test all the products available (more than 200 for dewatering applications), a primary screening will do. Select the ionic charge by testing a range of products with the same molecular weight: FLOPAM™ AN 900 SH Series for the anionic pow-ders and FLOPAM™ FO 4000 SH for the cationic powders.

4.2.2. Laboratory equipment:

In order to analyse the volume of drained water in function of time, the minimum equipmentnecessary is as follows:● 400 ml beakers● 90 mm diameter buchner funnel.● 90 mm diameter belt filter cloth● 250 ml graduated cylinder● Stopwatch

A repeatable drainage control can be done via a computer-linked scale that weighs the drainedfiltrate. This computer recording of the drainage test is recommended since the first 10seconds of the drainage are the most crucial.

Note:See the brochure « Preparation of Organic Polymers » to have a list of the necessary equip-ment to prepare polymer solutions.

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4.2.3. Test procedures:

The aim of this chapter is not to impose a testing procedure but to describe the key elements that are essential and common to all the existing procedures. The selection of thepolymer(s) best adapted to belt filter dewatering is done in two steps.

● Selection of the ionic charge:Polymers having the same molecular weight and different ionic charges are compared atdifferent dosing levels (low, optimal, and high).

The determination of the optimal polymer dosage must first be made. To do so, a mediumrange polymer is tested on the sludge by beaker to beaker pouring. The initial dosage is func-tion of the sludge concentration (ie: 5ml of a 3g/l solution in 200ml of 30g/l sludge). If a good floc-culation occurs at this dosage, the test is repeated at a lower dosage (ie: - 1ml). If the floccu-lation is bad or doesn’t occur, increase the initial dosage (+ 1ml). The minimal dosage is thelowest dosage that still flocculates.

Then other polymers are tested starting with the minimal dosage determined above and thesame number of beaker to beaker pours.

The interpretation of the results can be based on two criteria:● The best flocculant is the one that frees the most water in minimum time.● The quality of the filtrate has also to be taken into account.

● Selection of the molecular weight:Flocculants having the same ionic charge but of different molecular weights are then testedusing the same method. The number of beaker to beaker pours is indicative of the capacityof the polymer to mix with the sludge.

4.2.4. Parameters to check and analysis of the results:

The key parameters to check are:

● The drainage speed during the first 10 seconds● The filtrate quality● The efficiency of the polymer at low, optimum and high dosages● The mixing ability of the polymer in the sludge.

All these data are then compiled in a spreadsheet in order to analyse the results in view ofthe selection criteria chosen.

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T E R I N G

4.3.1. Setting up a trial:

During the trial period the quality and flow of the effluent must be as typical as possible.The making up of the reagents must be thorough (See the brochure « Preparation ofOrganic Polymers »). The concentration of the reagents, the injection points… must be selec-ted based on the lab results.

4.3.2. Parameters to follow and analysis of results:

The main parameters to follow depend on the results targeted by the operator. Neverthelessthe most frequent are:

● Flocculant Mass Flow (kg/h, lbs/HR)● Sludge Mass Flow (kg/h, lbs/HR)● Sludge Concentration (g/l, %)● Injection points● Equipment parameters: belt speeds, flocculator speed, pressure● Floc Size● Drainage● Dryness of the cake (%)● Filtrate quality (g/l, mg/l of Suspended Solids)

To analyse these results, it is usefull to compile all the data in a spreadsheet in order to calculate the chemical consumption rates per volume treated or per dry solids tonne (DST). This spreadsheet also allows the calculation of perfomance costs.

4.4. Optimizing belt filters:

The quality of the sludge flocculation plays a major role in the end results: sludge flow, filtratequality, cake dryness. On belt filters, it is easy to check the quality of the flocculation in thedrainage zone.The three main operating problems are:

● Bad drainage: The sludge reaches the progressive compression zone without being fullydrained.● Sludge creep: In the compression zone, the sludge has a tendency to run away on thesides of the belt.● Low cake dryness: The sludge cake’s Dry Solids content is too low.

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4.3. Plant trials:

Industrial trials are used to confirm the lab test selection.

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4.4.1. Bad drainage:

When the drainage is insufficient, the following parameters must be checked:

Mixing conditions: The mixing of the sludge with the flocculant solution must be optimal toobtain the best floc size. To achieve this, it is necessary:

● To have a good mixing intensity: speed of the flocculator.● To determine the best injection point: before or after the sludge pump or in the floccu-lator or having several injection points…● To check the sludge distribution on the belt.

Belt cleaning: If the belts themselves are not perfectly clean, it is impossible to have agood drainage since the pores are plugged. So the following parameters have to be checked:

● The cleaning water flow may be too low● The cleaning water pressure may be too low● The cleaning nozzle may be blocked

The belt pressure can also influence pore plugging. The higher the pressure the more sludgegoes through the belt and dirties it.

Flow: If the flocculation quality is not optimal, drainage will be low. Adjusting the sludgeflow and the flocculant flow must be done. The flocculant post-dilution is also an importantstep; it allows for a better dispersion in the sludge.

4.4.2. Sludge creep:

Sludge creep (squeeze out) happens frequently on biological sludge which are difficult todewater and sensitive to pressure. Three parameters must then be modified:

Flocculation: Optimal dosage is necessary for the best results. In order to determine theoptimal dosage, lower to the minimal dosage and slightly increase it. Structured polymerscan be used to obtain a better drainage and a better floc structure.

Drainage: The faster water is released, the dryer the sludge is when reaching the com-pression zone. Checking the initial drainage in order to obtain a maximum water release andchecking the cleaning of the belts should resolve this problem.

Sludge feeding: A high Solids Content can induce creep. Lowering of the sludge flow,reducing the width of the drainage zone and optimizing the cake thickness are the parame-ters to check.

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T E R I N G

4.4.3. Low cake dryness:

It is sometimes possible to have a better dryness when modifying the following parameters:

● Mixing conditions● Belt speed and tension● Polymer selection

Mixing conditions: If mixing is not good, dryness may be low. Rearrange the mixingconditions and the injection points.

Belt speed and tension: If the belt speed is high, the drainage time is short. By reducingthe speed, the drainage time is longer and therefore the drainage is better. The belt pressureis an important factor to get a good dryness. By increasing the belt pressure a better dewatering is obtained.

Polymer selection: An accurate polymer selection will give the best results. Changing themolecular weight, the structure of the polymer and its dosage; all these parameters,although preselected in the lab tests, can be optimized on a plant scale.

Cake thickness: The cake thickness should be adjusted taking into account the intrinsic characteristics of the sludge.

4.4.4. Summary of the adjustable parameters:

Note: shows an increase of the parameter and indicates the trend in dryness or in

Suspended Solids. For example, when the sludge flow increases, the dryness decreases andthe Suspended Solids in the filtrate increase .

SLUDGEFLOWPARAMETERS

POLYMERFLOW

BELT SPEED

BELTTENSION

CAKEDRYNESS

SUSPENDEDSOLIDS

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5Centrifuge dewatering5

Centrifuges dewater continuously using centrifugal forces of several thousand gs.

5.1. Equipment description and operating principle:

The principle of a centrifuge, also known as centrifugal decantor, is to use centrifugal forceto accelerate solid-liquid seperation.To simplify, it can be assumed that a centrifuge is a conical cylinder decantor that turns hori-zontally on its axis with a clarified water overflow, the dewatered sludge being removed by anArchimedean screw. The rotation applies a centrifugal force on the solid particles that thenmove a lot more quickly.

In practice, the flocculated sludge is injected inside the centrifuge bowl through an injectionpipe. The bowl has a high rotation speed (3500 rpm) and the particles are flattened againstthe bowl’s sides in the clarification zone. The particles are then pushed by an Archimedeanscrew towards the end of the bowl’s cone in the sludge spin-dry zone. The clarified liquid called, centrate, is evacuated at the other end of the bowl by overflow.

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S L U D G E D E W A T E R I N G

Filtrate or centrate1

1

1

Sludge cake2

2

Bowl3

3

Screw4

4

Sludge+

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T E R I N G

Several parameters, specific to centrifuge dewatering, should be considered since they affect on the polymer selection during lab tests and industrial trials.● Diaphragm diameter:This parameter corresponds to the diameter of the centrate exit hole. It is controlled by aseries of plates that can be adjusted in height. Inside the bowl a liquid ring is created, itsthickness is equal to the distance between the bowl’s surface and the edge of the plate.The smaller the diaphragm diameter, the thicker the liquid ring.The diaphragm diameter is selected to obtain the best compromise between centrate clarification and sludge dryness.● Relative speed: The relative speed is the difference in rotation speeds between the bowl and the centralArchimedean screw. The higher the relative speed, the faster the sludge is extracted.● Torque:The torque measures the pressure of the sludge on the screw. This pressure creates torsionon the screw axis. This torsion is measured to give the torque. The higher the sludge pressureon the screw, the higher the torque.

Note: This is only one example of torque measurement.

Torque and relative speed are linked. An increase of the relative speed will lower the torqueand the sludge is extracted more quickly.

High Performance Centrifuges increase sludge dryness. They have a different structure forthe conveying screw that allows for a longer residence time in the centrifuge.

5.2. Lab tests:

5.2.1. Sampling:

Of the solution to be treated: A representative sample of the sludge must be taken. In order to achieve this, the samplewill be taken just before the polymer injection point and the lab tests will be executed rapid-ly after sampling (the sludge condition may change over time).An analysis of the Dry Solids content (DS) of the sludge must be made since the polymerdosage is a function of the DS.

Of the polymer: It is not necessary to test all the products available (more than 200 for dewatering applica-tions), a primary screening will do. Select the ionic charge by testing a range of products withthe same molecular weight: FLOPAM™ AN 900 SH Series for the anionic powders and FLO-PAM™ FO 4000 SH for the cationic powders.

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5.2.2. Laboratory equipment:

In order to analyse the volume of drained water in function of time, the minimum equipmentnecessary is as follows:

● 400ml beakers● Mechanical mixer with blade● Stopwatch

Note:See the brochure "Preparation of Organic Polymers" to have a list of the necessary equip-ment to prepare polymer solutions.

5.2.3. Test procedures:

The aim of this chapter is not to impose a testing procedure but to describe the key elements that are essential and common to all the existing procedures. The selection of thepolymer(s) best adapted to centrifuge dewatering is done in two steps.

● Selection of the ionic charge.Polymers having the same molecular weight and different ionic charges are compared atdifferent dosing levels (low, optimal, and high).The determination of the optimal polymer dosage must first be made. To do so, a mediumrange polymer is mixed with the sludge by the mechanical mixer. The initial dosage is a function of the sludge concentration (ie: 5ml of a 3g/l solution in 200ml of 30g/l sludge). If agood flocculation occurs at this initial dosage, the test is repeated at a lower dosage (ie: - 1ml). If the flocculation is bad or doesn’t occur, increase the initial dosage (+ 1ml). The minimaldosage is the lowest dosage that still flocculates.Then other polymers are tested starting with the minimal dosage determined above +15%.

The interpretation of the results can be based on two criteria:● The best flocculant is the one that gives the biggest flocs after the longest mixing time.● The best flocculant must reflocculate the best (add 20% of the initial dosage after com-pletely breaking the flocs).

● Selection of the molecular weight.Flocculants having the same ionic charge but of different molecular weights are then testedusing the same method.

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T E R I N G

5.2.4. Parameters to check and analysis of the results:

The key parameters to check are:

● Floc shear resistance● Reflocculation● Centrate quality● Incorporation

All this data is then compiled in a spreadsheet in order to analyse the results in view of theselection criteria chosen.

5.3. Plant trials:

Industrial trials are used to confirm the lab test selection.

5.3.1. Setting up a trial:

During the trial period, the quality and flow of the effluent must be as typical as possible.The making up of the reagents must be thorough (See the brochure "Preparation of OrganicPolymers" ). The concentration of the reagents, the injection points… must be selected basedon the lab results.

5.3.2. Parameters to follow and analysis of results:

The main parameters to follow depend on the results targeted by the operator. Neverthelessthe most frequent are:

● Flocculant mass flow (kg/h, lbs/HR)● Sludge mass flow (kg/h, lbs/HR)● Concentration of the sludge (g/l, %)● Injection point● Equipment parameters: torque, relative speed● Dryness (%)● Centrate quality (g/l, mg/l of Suspended Solids)

To analyse these results, it is useful to compile all the data in a spreadsheet in order to calculate the chemical consumption rates per volume treated and per Dry Solids Tonne(DST). This spreadsheet also allows the calculation of perfomance costs.

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5.4. Optimizing centrifuges:

It is a lot more difficult to control the flocculation quality on a centrifuge than on a belt filtersince everything takes place inside pipes or inside the machine. Within the centrifuge, theflocs are submitted to an important force (2000g to 4000g). The flocs can be quickly destroyed if the polymer has not been well chosen or dosed.The three main operating problems are:

● Black centrate: The centrate has an unusual dark color. This color is due to the presenceof numerous particles that are evacuated with the diaphragm overflow and are going back tothe start of the process.● Gray, foaming centrate: Only an excessive foaming is indicative of a problem. Slight foamingis a normal occurrence due to the high agitation level inside the centrifuge. Degasing eliminatesthis foam.● Low dryness: The sludge cake has a lower than expected solids content.

5.4.1. Black centrate:

When the centrate has too many solids, the following points must be checked:

Solids flow: Each centrifuge is designed for a specific solids flow load. If it is overloaded, thecentrifuge will never work well.

Relative speed: The lower the relative speeds, the longer the residence time of the sludgein the centrifuge. The pressure applied to the flocs will have time to destroy them and thefine particles will exit with the centrate. Increasing the relative speed should clarify the centrate.

Polymer flow: The flocs have to be solid enough to resist the shearing generated insidethe centrifuge by the pressure and the screw. The dosage of the polymer should be highenough to reflocculate the particles if there is a destruction inside the centrifuge.In any case, the dosages of polymer for a centrifuge application is always higher than for belt filters.

5.4.2. Gray, foaming centrate:

In this case it is necessary to adjust:

The mass flow: A lack of solids forces the centrifuge to reduce its relative speed in orderto maintain a sufficient torque. It is then necessary to adjust the polymer dosage to the qualityof sludge.

The torque: If the torque is unstable, the centrifuge cannot work correctly and particles willappear in the centrate during the changes in relative speed.

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The polymer flow : The presence of foam may indicate polymer overdosing.Furthermore, polymer overdosing redisperses the particles. Reducing the polymer dosagemust be tested.

Note :A gray color may indicate that the polymer is not well adapted to the sludge, due to a wrongpolymer selection and/or a bad mixing of the polymer with the sludge.

5.4.3. Low cake dryness :

Adjusting the following parameters may enhance the cake dryness:

Torque : Adjusting the torque and/or the relative speed to the sludge concentration mustbe systematic.

Liquid ring : A smaller liquid ring diameter ensures a better drying of the sludge sincethe water height in the conical part of the centrifuge is lower.

Floc stability : Floc stability is a key parameter in the performance of a centrifuge becauseof the high shear forces involved. SNF has developed a range of emulsion cationic flocculantsin order to provide the best stability and an optimum reflocculation.

5.4.4. Summary of the adjustable parameters :

Note: shows an increase of the parameter and indicates the trend in Cake Dryness

or in Suspended Solids. For example, when the sludge flow increases, the Cake Drynessdecreases and the Suspended Solids in the filtrate increases

SLUDGEFLOWPARAMETERS

POLYMERFLOW

TORQUE RELATIVESPEED

CAKEDRYNESS

SUSPENDEDSOLIDS

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2Vertical hollow plates1Filter cloth2Jack3Filtration chamber4

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Frame filter presses will dewater sludge at a higher level of dryness than the previous techniquesreviewed above. It is a discontinuous process that works in batch cycles.

6.1. Equipment description and operating principle:

With a frame filter press, also called a filter press, the operating principle is filtration. A filter press is composed of a series of hollow vertical frames with filter cloths stretched onboth sides. These frames are hung next to each other and pressed together with a hydraulicjack. A filter chamber is formed bewteen the plates.

Each cycle can be split into three phases:

Filling phase: At the start of the cycle, the conditioned sludge is injected in the filtrationchambers by a high-pressure pump. The sludge fills each chamber and the water starts toseep out.

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Filtration phase: Once all the chambers are filled, the sludge continues to be pumped inand the pressure increases to reach up to 15 bars. The filtrate flows into the chanels placed ineach frame and is evacuated in a main pipe. The sludge injection flow reduce when the pressureincreases. Very often, two seperate off-centre-screw pumps are used: a high flow/low pressure pump for the beginning of the cycle and a low flow/high pressure pump for the end.

Opening phase: Several parameters may be used to signal the end of the cycle (stoppingthe injection pump): maximum pressure, filtration time, filtrate volume. Once the press hasstopped, the central core is purged of the liquid sludge inside. The jack that presses theframes together is released. The chambers are opened sequentially and the cake falls belowinto a skip or on to a conveyor.

Membrane Filter Presses have been recently developed to obtain higher cake dryness thanon regular Frame Filter Press. Every second plate is made of a membrane that can be shapedby air or water pressure (7-10 bars). This extra pressure is applied at the end of the injectionphase, once the cake is formed.

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Sludge pressure injection5Filtrate evacuation6Compressed air7

Opening phase and cake falling8

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6.2. Lab tests:

6.2.1. Sampling:

Of the solution to be treated.A representative sample of the sludge must be taken. In order to achieve this, the samplewill be taken just before the polymer injection point and the lab tests will be executed rapidlyafter sampling since the sludge condition may change over time.An analysis of the Dry Solids content (DS)of the sludge must be made since the polymer dosageis a function of the DS.

Of the polymer: For Frame Filter Press several combinations may be envisaged:

lCombination of iron salts and lime (the most frequent). This method significantly increasesthe volume of sludge to be treated (30%-50% of extra mineral matter).

lCombination of iron salts and cationic flocculant

lOrganic coagulant alone

lOrganic flocculant alone

6.2.2. Laboratory equipment:

In order to control the floc resistance under pressure, the minimum equipment necessary isas follows:

l400ml beakers

lMechanical mixer with blade

lStopwatch

lCapillary Suction Time (CST) meter

Note:See the brochure "Preparation of Organic Polymers" to have a list of the necessary equip-ment to prepare polymer solutions.

6.2.3. Test procedures:

The aim of this chapter is not to impose a testing procedure but to describe the key elements that are essential and common to all the existing procedures. The selection of thepolymer(s) best adapted to frame filter press dewatering depends on the conditioningmethod employed.In the case of an organic flocculant alone: the method will be similar to Belt Filters andCentrifuges.

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Selection of the ionic charge.Polymers having the same molecular weight and different ionic charges are compared atdifferent dosing levels (low, optimal, and high).The determination of the optimal polymer dosage must first be made. To do so, a mediumrange polymer is mixed with the sludge by the mechanical mixer. The initial dosage is a functionof the sludge concentration (ie: 5ml of a 3g/l solution in 200ml of 30g/l sludge). If a goodflocculation occurs at this initial dosage, the test is repeated at a lower dosage (ie: - 1ml). Ifthe flocculation is bad or doesn’t occur, increase the initial dosage (+ 1ml). The minimal dosa-ge is the lowest dosage that still flocculates.Using Capillary Suction Time (CST) allows to evaluate the size of the flocs versus mixing time.The best flocculant is the one that gives the shortest CST times after the longest mixing time.

Note: Never do a CST on flocs that are too big or on suspensions that are not homogenous(solid-liquid separation in the beaker).

Selection of the molecular weight.Flocculants having the same ionic charge but of different molecular weights are then testedusing the same method.

When a combination of mineral/organic coagulant and an organic flocculant is used, it isnecessary to first determine the optimum coagulant dosage using the CST and then to selectthe optimum flocculant using the method described above.

6.2.4. Parameters to check and analysis of the results:

The key parameters to check are:

lFloc size and floc resistance measured with the CST

lFiltrate quality

lIncorporation

All this data is then compiled in a spreadsheet in order to analyse the results in view of theselection criteria chosen.

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6.3. Plant trials:

Industrial trials are used to confirm the lab test selection.

6.3.1. Setting up a trial:

During the trial period the quality and flow of the effluent must be as close to the averageas possible. The making up of the reagents must be thorough (See the brochure"Preparation of Organic Polymers"). The concentration of the reagents, the injectionpoints… must be selected based on the lab results.

6.3.2. Parameters to follow and analysis of results:

The main parameters to follow depend on the results targeted by the operator. Neverthelessthe most frequent are:

lReagent mass flow (kg/h, lbs/HR)

lSludge mass flow (kg/h, lbs/HR)

lConcentration of the sludge (g/l, %)

lPressure build-up

lFiltration cycle time

lCake aspect

lCake dryness (%)

To analyse these results, it is useful to compile all the data in a spreadsheet in order to calculate the chemical consumption rates per volume treated and per Dry Solids Tonne(DST). This spreadsheet also allows the calculation of perfomance costs.

6.4. Optimizing frame filter press:

For a frame filter press, the most frequent conditioning by far was the combination of FeCl3

and lime. Today, due to the ever-increasing volume of sludge to be disposed of, any unnecessary increase in sludge volume leads to unacceptable operating costs (too many opera-ting cycles and too much sludge volume). That is the reason behind the new organic poly-mer conditioning processes that have been developed.

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The three main operating problems in an organic polymer conditioning process are:

lSticky cakes: This is the main problem when using organic polymers. Instead of falling off bygravity, the cake sticks to the filter cloth and an operator has to intervene during the opening phase.

lCloth plugging: Pores get plugged, preventing the filtrate from passing through the cloth.

lPolymer efficiency loss: This phenomenon is often observed when the liquid sludge hasbeen previously treated with lime. It results in a degradation of the flocs with time.

6.4.1. Sticky cakes:

When the cake starts to stick the following parameters must be checked:

Dosage of the chemicals.When a cationic flocculant is used, the sticky cakes may be due either to an overdosing or toan underdosing. The association of FeCl3 and a flocculant is best suited to remedy this problem,especially on sludge with a high VSS. The addition of FeCl3 allows a lower the flocculant dosa-ge.

Cloth cleanliness.If the cloth is clogged, it is impossible to get a good dewatering. Regular cleaning of the cloth(every 10-30 cycles) is necessary.

Polymer selection.If the flocculant is not well adapted (cationicity, molecular weight, dosage), the cake maystick. It is necessary to check in the lab the dosage and the molecular weight.

6.4.2. Cloth plugging:

This problem is signaled by a rapid increase in pressure. The pressure increase during the cyclemust be slow and regular. This parameter is used to check the degree of advancement of thecycle.A regular sampling between the sludge pump and the filter plates will permit a check with theCST meter of the strength of the flocs submitted to pressure.

Other parameters also to consider:

Cloth cleanliness. The more the pores are plugged, the faster the cloth will clog. Cleaningthe cloth with high-pressure water must be done.

Floc size: Optimum filtration is obtained with small pressure-resistant flocs. To get thesetype of flocs, the best flocculants are often medium to low molecular weights and structured(branched and cross-linked).

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6.4.3. Polymer efficiency loss:

In this case the following parameters must be checked:

Polymer preparation.Degradation in time of a cationic polymer is a normal phenomenon (see paragraph on hydro-lysis in the brochure"Preparation of Organic Polymers"); there is a loss in viscosity and in per-formance of the polymer. Checking the polymer solution (age, concentration, pH…) is recom-mended.

Sludge pH. If the sludge to be dewatered has a high pH (10/11), as is likely with the addition of lime, thecationic flocculant will hydrolyse very quickly. This hydrolysis can be easily observed since theflocs will break at the slightest movement. There are two ways to solve this problem: changethe sludge pH or choose a polymer less sensistive to hydrolysis like the FLOPAM™ CB Seriesor polyDADMACs.

Sludge-polymer contact time.Under difficult operating conditions such as a high pH, the cationic polymer is not stable for along time. Selecting the injection point will become an important factor to the performance.The best conditions are always in-line injection with the polymer dosing regulated by thesludge flow.

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SNF has compiled this hanbook to the best of its knowledge.For any further enquiries please contact SNF on www.flocculant.com.

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Notes :

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The information in this brochure is provided in good faith. To our knowledge it reflects the truth.

SNF FLOERGERZAC de Milieux

42163 Andrézieux Cedex - FRANCETel: + 33 (0)4 77 36 86 00Fax: + 33 (0)4 77 36 86 96

[email protected]

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